Now, being able to play dead elvis would guarantee my subscription for some considerable time.

Yes. Sub-races was my early prediction for Mists (that is, before it was announced). I was wrong; Blizzard went with pandas, but I still think it's a great idea.

Sub-races give people lots of new and interesting choices. But Blizzard could still keep their development costs down by making all sub-races use the same skeleton and animations.

I predict it (again) for the next expansion after Mists.

While I would like it I think it would unbalance the racial representatives a lot.
take Dwarves for instance, they are mostly Bronzebeards in lore, the other 2 races only recently returned to Ironforge.
yet I feel a lot of players would prefer the tatooed Wildhammer or the Red eyed Dark Iron over the more bland Bronzebeards.

This especially goes for sub-races that really look different like Taunka, Broken Draenei, Undead Elves or Forest Trolls.
And what about races that don't have visually different subraces like Humans, Night Elves or Worgen?